1. Field of the Invention
The present invention relates to a feedstock particularly adapted for use in semi-solid metal injection molding. More specifically, the present invention relates to a feedstock that more easily forms its liquid phase. As such, the feedstock forms its liquid phase at lower temperatures, with lower thermal gradients, less plugging and with less thermal shock in the initial zones of the semi-solid metal injection molding machinery. This in turn allows for faster feed rates, flood feeding of the feedstock, longer barrel life, less down time, less energy usage, superior molded parts and lower operating costs.
2. Brief Description of the Prior Art
Generally semi-solid metal injection molding is the process whereby an alloy feedstock is heated, subjected to shearing and injected under high pressure into a mold cavity. Heating brings the feedstock into a state where both solid and liquid phases are present while the application of shearing forces prevents the formation of dendritic structures in the semi-solid alloy. In this state, the alloy may exhibit thixotropic properties. It is to such alloys that the present invention is applicable.
The feedstock may be received into the barrel of the semi-solid metal injection molding machinery in one of three forms: liquid, semi-solid or particulate solid. The former two forms require additional equipment and special handling precautions to prevent contamination of the alloy material and therefore increase costs. The latter form, while being more easily handled results in longer cycle times and significant thermal gradients in the first encountered portions of the barrel and more pronounced thermal shock to that portion of the barrel. A solid feedstock which does not result in the above conditions is therefore seen as desirable.
More specifically, semi-solid metal injection molding (SSMI) involves the feeding of alloy feedstock into the barrel of the semi-solid metal injection molding machinery. In the barrel, the alloy feedstock is heated and subjected to shear, often by a screw located therein. As a result of heating and shearing, the temperature of the alloy feedstock is raised above its solidus temperature to a temperature below its liquidus temperature. Within this temperature range, the feedstock is transitioned into semi-molten material having co-existing solids and liquid phases. In addition to aiding to heating, shearing further prevents the formation of dendritic structures in the alloy. In this thixotropic state, the semi-solid alloy material is injected, either through a reciprocation of the screw or transfer to a shot sleeve, into a mold cavity and solidified to form the desired part.
U.S. Pat. No. 4,694,881, 4,964,882, and 5,040,589, issued to The Dow Chemical Company, describe methods for semi-solid metal injection molding and an apparatus for performing the above process. These patents are herein incorporated by reference.
In conventional preparation of particulate feed stock, an ingot or billet is initially formed from the alloy, cooled and then mechanically chipped to provide particulates of the appropriate size. Notably, after the initial formation of the ingot or billet, cooling is effectuated slowly thereon. Magnesium alloy such as AE42 and aluminum alloy such as A356 are available in the above form.
As mentioned above, in carrying out the semi-solid injection molding process, use of conventional alloy feedstock results in the initial portion of the barrel, into which the feedstock is first received, being subjected to highly cyclic thermal loads in order to initiate the conditioning of the feedstock (while the exterior of this portion of the barrel remains highly heated, the interior is significantly cooled upon the influx of each new change of feedstock). As a result of the high thermal gradient therein, this portion of the barrel experiences high thermal stresses.
The common characteristic of the above type of alloy feedstocks is that, upon review of a differential scanning calorimetry (DSC) curve, it is noted that the alloy feedstocks exhibit a sharp and vigorous absorption of energy during initial melting temperatures. This sharp energy requirement over a narrow temperature region places an abnormal heating demand on the barrel in a short region which therefore sees high temperature gradients (between the barrel's inner and outer surfaces) and high thermal stresses. Since as much as approximately fifty percent of the melting occurs within 30.degree. C. of the solidus temperature of the low melting point constituent, if advancement of the material within the barrel is not precisely controlled, this pronounced sensitivity to a small temperature change can result in freezing of the material within the barrel as a plug forms around the screw. When such freezing and plug formation occurs, good parts can no longer be produced. It requires pulling the screw and the time consuming operation of cleaning the screw and barrel, at a significant cost and loss of production. If freezing and plug formation do not occur, the necessary time for heating the material to the appropriate molding temperatures limits feed rates and cycle times for the machinery.
In view of the above and other limitations, it is an object of the present invention to provide a particulate feedstock that forms its liquid phase more easily allowing for faster feed rates and decreased cycle times for the semi-solid injection molding machinery. Additionally, an object of the present invention is to provide a feedstock that allows for lower barrel temperatures, decreased thermal gradients through the barrel wall, and less thermal shock on the barrel. A further object of the present invention is to provide a feedstock which will allow for the presence of a small percentage (five to twenty percent) of the alloy's initial liquid phase in the first heating zone of the machine thereby improving heat transfer to the remaining constituents of the alloy in the subsequent heating zones of the barrel. Another object of this invention is an alloy feedstock whose DSC curve generally follows the temperature profile of the barrel over the barrel's length, thereby reducing thermal gradients and shock in the barrel. One feature of the present invention is therefore the ability to mold alloys that have a higher solidus temperature than alloys conventionally used in semi-solid molding.